N-methyl-D-aspartate receptors (NMDARs) play critical roles in information processing and in the synaptic plasticity that underlies learning and memory. Depending upon the pattern of activation, NMDARs can promote long-term potentiation (LTP) or long-term synaptic depression (LTD), two leading candidates for synaptic memory mechanisms. When activated excessively, however, NMDARs can also cause several forms of neurodegeneration. We have observed that there are also conditions in which NMDAR activation produces no change in synaptic responses or neuronal injury, but impairs the ability to generate LTP. This NMDAR-mediated LTP inhibition is observed with low level activation of NMDARs, certain patterns of synaptic stimulation and exposure to sub-lethal stressful conditions (brief hypoxia and low glucose). Because of the role that synaptic plasticity plays in memory processing, this NMDAR-mediated LTP inhibition may be important for understanding the cognitive defects that accompany untimely NMDAR activation in neuropsychiatric disorders. In recent studies, we found that low level NMDAR activation promotes the production of GABA-enhancing neurosteroids in hippocampal pyramidal neurons and these neurosteroids play a key role in LTP inhibition. In this proposal, we will extend our work on NMDAR LTP inhibition by pursuing three aims: 1. To determine conditions under which NMDAR activation promotes neurosteroid production and how steroids contribute to LTP modulation; 2. To determine signaling mechanisms underlying NMDAR-induced neurosteroid production; and 3. To determine the effects of NMDA and neurosteroids on GABAergic inhibition and the mechanisms underlying these effects. These studies will be conducted in the CA1 region of rat hippocampal slices, an area known to be important for memory processing. Our long-term goal is to identify ways to preserve and restore synaptic function in individuals with neuropsychiatric illnesses.